The present invention relates to a method for the preparation of a low-impurity organopolysiloxane by the polymerization of an organopolysiloxane oligomer or, more particularly, to a method for the preparation of a high-polymeric organopolysiloxane outstandingly free from impurities such as alkalis, acids and halogens by the alkali-catalyzed polymerization of an organopolysiloxane oligomer.
As is well known, high-polymeric organopolysiloxanes are prepared usually by the polymerization of an oligomeric organopolysiloxane or a mixtures of oligomeric organopolysiloxanes obtained by the hydrolysis of an organohalosilane or a mixture of organohalosilanes followed by the dehydration condensition. The polymerization of the oligomeric organopolysiloxane proceeds by the siloxane rearrangement in the presence of an alkali catalyst. The alkali catalyst must be neutralized after completion of the polymerization since otherwise even a trace amount of alkali may adversely act on the organopolysiloxane product to cause depolymerization thereof. Various compounds have been proposed as the neutralizing agent of the residual alkali catalyst including, for example, ammonium halides proposed in U.S. Pat. No. 3,057,821 and .alpha.-halohydrins proposed in U.S. Pat. No. 3,065,203.
These neutralizing agents, however, have their respective disadvantages and problems. For example, the ammonium halides are hardly miscible with organopolysiloxanes so that the neutralization reaction cannot be complete. Organopolysiloxanes obtained by the neutralization with an .alpha.-halohydrin sometimes suffer from poor heat stability and corrosiveness against metals in contact therewith due to the significantly high level of the halogen impurities originating from the neutralizing agent.
Along with the recent development in the electronics technology, the organopolysiloxane compositions used in the protective encapsulation and insulating sealing of the electronic devices and instruments are required to be as free as possible from any impurities not only of the alkalis used as the polymerization catalyst but also of acids and halogens originating in the neutralizing agent in order to ensure high and durable performance of the device and instrument. In this regard, none of the hitherto proposed neutralizing agents is quite satisfactory.